Gas diffusion and distillation

9. February 2010 by The Cyanide Guy.

perhaps I have been unclear, or expecting everyone to understand exactly what gas diffusion is. To explain this further let’s first differentiate between the concept of manual distillation of cyanide versus the automated distillation that occurs on an automated analyzer. They are different.

Manual Distillation is not really a distillation as you may interpret it. While you may be heating the solution to boiling you are not passing the solution from container A and condensing it in container B. In cyanide manual distillations you are boiling the sample in strong acid for a prolonged period of time (about 1 hour usually). A cold finger condenser drips the steam back into the original container as water. A carrier stream of laboratory air bubbles through the boiling sample and pulls the vapor (theoretically the HCN) into an impinger containing a sodium hydroxide solution. It is the prolonged boiling with strong acid that decomposes the cyanide metal bonds, allows the CN to convert to HCN, The boiling (heat) combined with the purging of air adds enough energy so that the HCN gas pass into the carrier stream. Cyanide will not transfer readily into the carrier stream without the heat. However, without the prolonged boiling in strong acid the metal cyanide complexes do not convert to cyanide.

In a Continuous Flow automated cyanide method the “distillation” is almost instantaneous. A portion of the sample is vaporized and recollected after passing through a condenser that converts the steam to water. There is no prolonged boiling with acid, and the process is too fast to decompose strong metal cyanide complexes. Thus, a continuous flow in-line distillation method is only capable of analyzing weak to moderate metal cyanide complexes without extra energy supplied.

These weak to moderate metal cyanide complexes are exactly the same cyanide species that are measured by gas diffusion. Instead of the rapid distillation where a thin film of liquid hitting a hot surface converts immediately to steam, the HCN generated upon acidification by acid passes through a hydrophobic membrane and is absorbed into a dilute sodium hydroxide solution. It is the exact same process, and the same cyanide complexes, simply separated from the acidified solution in a slightly different way.

So if both flash distillation and gas diffusion separate the exact same weak to moderately strong metal cyanide complexes, how do they analyze for total cyanide (strong metal cyanide complexes)? The answer is by UV irradiation. All continuous flow methods for total cyanide require UV irradiation to liberate total cyanide so that it can be separated from the matrix by flash distillation or gas diffusion.

I hope you can see that the flash distillation adds nothing extra to the analysis except extra complexity. A flash distillation requires heat and a condenser. The heat requires an extra module and the condenser requires flowing water. All these extras are not recovering more cyanide. Total is dissociated by UV irradiation and all other cyanide complexes are equally recovered by gas diffusion or flash distillation. 

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